In conventional turbofan gas turbine engines having a core engine, bypass duct and an augmentor, or afterburner, variable are bypass injectors (VABIs) are sometimes utilized to unload the engine fan at certain operating conditions for reducing back pressure and thereby maintaining desirable fan stall margin. The VABI system also provides a method for adjusting the engine cycle. For example, a conventional rear VABI may be used and is located between the core engine and the augmentor for selectively diverting some of the bypass airflow being channeled through the bypass duct into the core stream discharged from the core engine. The augmentor includes a bypass channel surrounding an augmentor outer liner which is disposed in flow communication with the bypass duct surrounding the core engine. Disposed radially inwardly of the outer liner is an inner liner which defines the augmentor combustion zone and bounds the combustion gases flowable therethrough. Disposed at the downstream end of the augmentor is a conventional variable area exhaust nozzle.
The bypass air channeled through the bypass duct surrounding the core engine, is channeled, in part, through the augmentor bypass channel and to the exhaust nozzle for providing cooling air to the exhaust nozzle. A portion of the bypass air is conventionally channeled between the augmentor outer and inner liners to provide cooling air to the inner liner. The inner liner may include conventional film cooling holes, or may be imperforate and conventionally cooled by convection from the bypass air channeled along its outer surface. A generally constant pressure drop across the augmentor outer liner is typically provided during the various modes of operation of the engine to ensure that acceptable cooling of the exhaust nozzle is obtained.
The prior art includes numerous types of valve assemblies for channeling bypass air in a gas turbine engine. The assemblies may include hinged flap valves or translatable ring valves for covering and uncovering apertures for controlling bypass flow. Various types of linkages are employed typically actuated by a plurality of actuators for positioning the valves. For obtaining acceptable operation of the valve assemblies, friction forces, flexibility, and operating clearances associated with the elements of the valve assemblies must be accommodated for providing control of the bypass air flow.
The prior art bypass valve assemblies vary in degree of complexity, weight, actuation force requirements and performance.